Coated magnetic agglomerates containing chromium dioxide



United States Patent Office 3,520,811 Patented July 21, 1970 3,520,811 COATED MAGNETIC AGGLOMERATES CONTAIN- ING CHROMIUM DIOXIDE Thomas J. Swoboda, Chester County, Pa., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del.. a corporation of Delaware No Drawing. Filed Nov. 13, 1967. Ser. No. 682,620 Int. Cl. HOlf 1/26; G03g 9/00, 7/00 US. Cl. 252-6254 13 Claims ABSTRACT OF THE DISCLOSURE FIELD OF THE INVENTION This invention relates to novel coated magnetic agglomerates, and more particularly to agglomerates containing chromium dioxide particles.

SUMMARY OF THE INVENTION The present invention comprises particles of magnetic material containing at least 5% of a chromium oxide in which the average valence of the chromium is greater than 3, said particles being coated and cemented to form agglomerates by an air-drying film-forming organic vehicle, preferably an air-drying alkyd resin.

The present invention also includes a method of making said cemented agglomerates which comprises (1) suspending the magnetic material containing at least 5% of a chromium oxide in particulate form in an inert organic medium, preferably an organic solvent, and subjecting the magnetic material to shearing agitation in the presence of anionic and non-ionic surfactants to form a suspension containing agglomerates of the magnetic material having substantially the desired size, (2) adding an airdrying film-forming organic vehicle to the suspension of agglomerates and agitating for a time sufiicient to coat the particles and bind said agglomerates.

This invention also includes stable magnetic toner compositions (in which the magnetic material can be magnetized) which comprise the aforesaid average agglomerates containing at least 5% of a chromium oxide in which the average valence of the chromium is greater than 3, coated with an alkyd resin, said particles being suspended in an ionizing liquid under basic conditions. Preferably the ionizing liquid medium is water at a pH of at least 8.0.

DETAILED DESCRIPTION OF THE INVENTION The present invention is based on the discovery that magnetic particles of chromium dioxide appear to catalyze a surface polymerization of organic air-drying filmforming vehicles, such as those employed in oil-based paints, so that a coating of polymerized vehicle is formed about the particle.

Suspensions of fine magnetic particles naturally tend to agglomerate by magnetic forces. The coating of polymerized vehicle which forms about the magnetic particles further cements the agglomerates together to form mechanically strong structures.

It has further been found that mixtures of particles of magnetic material containing even minor proportions of chromium dioxide particles can be coated and cemented into agglomerates by the process of the present invention, even though the same particles of magnetic material are not coated by the process of this invention in the absence of the chromium dioxide particles. The activity of the chromium dioxide particles in the mixed agglomerates is further indicated by the slower rate of coating when lesser proportions of chromium dioxide are present.

Yet further, it has been discovered that magnetic materials which are not coated alone by the process of the present invention can be coated if pre-treated with chromium trioxide solutions.

The eflFect described above of chromium dioxide in causing the polymerization of air-drying organic vehicles is surprising, since chromium compounds have not been known heretofore to catalyze the polymerization or accelerate drying of such vehicles.

Chromium dioxide throughout this specification and the appended claims, is intended to include the substantially pure material or chromium dioxide which has been modified with one or more reaction elements. Suitable descriptions of both the process of preparation and the compositions can be found in the following illustrative list of issued U.S. patents:

Arthur, U.S. 2,956,955

Arthur & Ingraham, 3,117,093

Cox, 3,074,778, 3,078,147, and 3,278,263

Ingraham & Swoboda, 2,923,683, 2,923,684, 3,034,988,

and 3,068,176

Swoboda, 2,923,685

Other magnetic materials in the form of fine particles which are capable of magnetization to a hard magnetic state or soft magnetic particles can be employed in the compositions of this invention as indicated above.

A number of factors contribute to the designation of a material as hard or soft magnetically. Many magnetic materials usually designated as soft will show high coercive force when prepared as fine particles. Geometrical factors, including size and shape of the particle, are important. For example, iron is normally considered a soft magnetic material with a coercivity of a fraction of an oersted. However, small iron particles composed of single domains with lengths great compared to their diameters can be expected to show coercivities of the order of 10 -10 Oe. In this case, high coercivity is due to shape anisotropy. For other materials, such as manganese bismuthide or cobalt, high coercivity for single domain particles may be the result of magnetocrystalline anisotropy arising from an easy direction of magnetization along a particular crystalline direction. Even fine nickel particles should show a high coercivity under uniaxial stress. Many normally soft magnetic materials not in single domain form can be made to exhibit a high coercivity after being subjected to cold work or other similar treatments designed to introduce defects or internal strains which serve to pin or block movement of domain walls. Further discussion of Hard Magnetic Materials can be found in the article by that title by E. P. Wohlforth, Advances in Physics, supplement to Philosophical Magazine, 8 (April 1959), pp. 87224, and in the book by R. M. Bozorth on Ferromagnetism, D. Van Nostrand and Company, Princeton, NJ. (1951), particularly the section on fine particles, pp. 828834. Soft Magnetic Materials are also discussed widely in the literature, e.g., E. W. Lee and R. L. Lynch, Advances in Physics, supplement to Philosophical Magazine, 8 (July 1959), pp. 292348.

Representative of such materials include the ferrites, the Alnicos, the Cunifes, etc., e.g., Mn FeTi O 3 Ml'lFfizOg, Fe 04, COFC204, NiFe O CUFGZO4, and Li Fe O the various Alnicos 1-8, Cunico, chrome steel, cobalt steel, Fe, Co, Ni and the Lodexes, 31, 32, 41, 42 and 55, as well as Fe Al, FeBe FeBe Fe B, 1763C, Fe Ce, Fe N, Fe3P, FeS, 'YF203, FCzP, PC3812, C 13, C08 CoZn, Co Zr, Ni Mg, Ni Mn, MnAs, MnB, MIlBi, MHQN, MIIP, MnSb, Ml'lzsb, Mnzsn, Cr S CrTe, Fe O S, AIFC204, COFCZO4, MgFe O MI1F204,

NiFfizOl;

La O -Fe og, Fe C or Fezncg iron carbide), Co P, Mn As Mn P C0 Fe O;,. (Co substituted 'yFe O (F,CO)2P, YCO5, M11 Al, F1 72Ge, BaO'6Fe O PtCo, Co modified Fe O and the like.

The magnetic material is employed in the form of a fine powder, which may be the result of the synthetic method employed in the manufacture, or may be achieved by mechanical attrition or other means. Suitable particles have a maximum dimension of from 0.01 to 5 microns and preferably from 0.1 to 2 microns. The particles can be substantially uniform in size, or can include a range of particle sizes.

In the process of the present invention the magnetic material is first dispersed in an inert liquid organic medium to form agglomerates of substantially the desired size which generally range from about 1.5 to times the maximum dimension of the individual particles. In order to assist the formation of uniform agglomerates, the organic liquid medium should have a high surface energy. Aromatic hydrocarbon liquids such as benzene, toluene and the xylenes and commercial mixed aromatic solvents are particularly suitable.

In order to obtain suitable agglomerates it is essential to employ an anionic surfactant, which greatly assists the dispersion of the magnetic material in the organic liquid medium. In some instances, the desired degree of agglomeration can be achieved by the use of an anionic surfactant alone. However, in most cases, anionic surfactants cause dispersion below the desired size of agglomeration. The particles can be re-agglomerated to the desired degree by the addition of a non-ionic surfactant. Thus in general a mixture of anionic and non-ionic surfactants is employed to control the degree of agglomeration.

Anionic and non-ionic surfactants are well-known in the art. Examples of suitable materials are given in the appended examples. Further materials are described in the Encyclopedia of Surface Active Agents by I. P. Sisly and P. J. Wood, The Chemical Publishing Co., Inc., New York, vol. I (1952), and vol. II (1964).

The dispersion of the agglomerates should be accomplished by agitation and shear, i.e., under conditions ordinarily employed in the art of paint mixing. Agitation in the presence of sand or pebbles is one method of agitation With high shear, but high speed blade mixers can also be used. Other methods of mixing with shear will occur to those skilled in the art.

As indicated above, the desired degree of agglomeration is substantially achieved in the absence of the air-drying organic vehicle, which primarily serves to coat the ultimate particles and bind the agglomerates together. However, many of the suitable vehicles also contain groups such as carboxylic acid groups which impart surface active properties to the vehicle. This is particularly true in the case of alkyd resins, which are preferred for many applications and which are essential for making stable inks containing magnetized particles. When such materials are employed as coating agents and binders some modification in the size of the agglomerates may occur, although the change is not generally major in character.

The state of agglomeration of the magnetic material prior to treatment with the air-drying film-forming organic vehicle can be determined readily by microscopic examination.

When the agglomerates have been dispersed to substantially the desired size, as described above, the airdrying film-forming organic vehicle is added. Generally the vehicle is thinned or dissolved in a small amount of the organic liquid medium to promote mixing prior to addition but this is not essential.

The air-drying film-forming organic vehicle which is added can be a simple unsaturated oil such as those described hereinafter for use in alkyd formulations or any other air-drying film-forming compositions which are typically employed in paints or coatings.

The preferred encapsulating agents are the drying alkyd resins (including alkyd resins which are classified as semi-drying). Alkyd resins of the drying type are generally the reaction product of a polyhydric alcohol, a polybasic (usually dibasic) acid and a drying. oil. Alkyd resins may be modified by reaction with rosin acids, rosin alcohols, epoxy compounds, phenolic resins, polyamides and isocyanates. Vinyl monomers such as styrene or acrylonitrile may also be included in the alkyd formulation.

Typical polyhydric alcohols which are employed are glycerol and pentaerythritol. Dibasic acids include phthalic acid, isophthalic acid with the possible addition of adipic acid, fumaric acid and maleic acid. Included in the oils are coconut oil, cottonseed oil, dehydrated castor oil, soya bean oil, linseed oil, tall oil, saffiower oil, soybean oil, tung oil, chinawood oil and acids derived by hydrolyzation of the above oils.

Catalysts to promote drying can also be employed in the practices of this invention.

The formation of alkyd resins is well-known to those skilled in the art. Details of the formulation and cooking of alkyd resins can be found in numerous texts such as Alkyd Resins by C. M. Martens, Reinhold Publishing Corporation, New York (1961).

The amount of alkyd resin, deposited on and encapsulating the particles of magnetic material depends on a number of factors, the most important of which are (i) The amount of chromium oxide in the magnetic material: the greater the proportion of chromium oxide present the faster the encapsulation process (ii) The weight of resin picked up increases with time (iii) The nature of the air-drying, film-forming organic vehicle; and

(iv) The greater the concentration of the vehicle the faster the rate of reaction.

Temperature, however, is not a critical variable and the time of reaction appears to be little affected by temperature. Ambient temperature is suitable for the process of this invention and is preferred for convenience. Temperatures in the range between about 10 to C. can be employed if desired.

The weight of organic material encapsulating the organic particles is important and critical to the successful manufacture of the cemented encapsulated agglomerates of the present invention. Below about 5 weight percent of resin on the surface of the particles, the particles are simply encapsulated and any agglomerates formed are re adily broken mechanically. Above about 10 Weight percent, the product is dominantly cemented agglomerates whose size is substantially that of the agglomerates of the initial dispersion step of the process, i.e., having a size of from about 1.5 to about 30 times the maximum dimension of the magnetic particles, and preferably between about 2p. and 30;! When more than 20 weight percent of resin is present on the particles secondary agglomeration occurs. Proportions of resin greater than 20 weight percent can be employed to form uniform agglomerates by the use of periodic high shearing conditions during the reaction to form the coated and cemented agglomerates.

For the above reasons, the weight percent of the resin should be from 5 to 25% and preferably from 10 to 20%.

DESCRIPTION OF THE PRODUCTS AND THEIR USES The coated and cemented agglomerates of the present invention have magnetic properties that are determined by the nature of the magnetic material encapsulated. The dry agglomerates form a free-flowing powder of low bulk density, generally less than 0.2 gm./cc. The agglomerates are characterized by an extremely uniform size, and have a generally spherical envelope within which their detailed surfaces, e.g., as viewed at high magnification under a microscope, are extremely irregular. When the powder is used as a toner for a developing magnetic image, as will be described more fully hereinafter, and the developed powder image is pressed onto paper, a remarkablly smear-free image is formed which is very superior to that formed by conventional carbon paper.

Magnetic pigments formed with chromium dioxide by the process of the present invention have an intense black coloration, which renders them particularly useful as inks and pigments.

When the encapsulating agent contains free acid groups, such as occur in the alkyd resin coatings, particularly remarkable, stable, magnetic inks can be made in which the magnetic particles can be magnetized without undesirable flocculation. These inks are formed by dispersing the particles in an ionizing liquid medium under basic conditions. In particular, water to which sufficient base has been added to give a pH of 8.0 or greater can be used to form magnetic inks. Surfactants and other ingredients may be added to the ink to improve wettability, adhesion to paper, and the like as will be a preciated by those skilled in the art.

The reason for the stability of the magnetized inks is not fully understood, however, it is believed that ionization of the acid groups in the encapsulating resin renders the agglomerates negatively charged in the liquid medium so that an electrical repulsion between the particles competes with the magnetic attraction, and hence flocculation of the magnetic pigment is significantly suppressed. The result is a highly modified manner of flocculation, observed under a microscope, in which the particles form a loose network of bead-like strings of individual particles, instead of the undesirable massive dense agglomerates observed in the absence of the electrical repulsion.

The magnetized toner of this invention has been found to decorate magnetic images on magnetic tapes and the like very rapidly and surprisingly very clearly. It is speculated that the magnetic field of the image is sulficient to break the bead strings and so transfer toner particles to the image while the forces maintaining the bead strings are suflicient to hold the strings intact adjacent to non-magnetic regions of the image, and the bead-strings are retained in the liquid phase by viscous forces. With massive agglomerates, i.e., in the absence of the electrical repulsion, the field of the image is not strong enough to break out toner particles nor rapidly attract or hold onto a whole massive agglomerate in the face of the viscous forces.

The uniform size of the cemented agglomerates of this invention render them particularly suited to the manufacture of reflex thermomagnetic recording elements.

For reflex thermomagnetic recording, a recording member is required which generally comprises a substrate film of a flexible, polymeric material on which is deposited a magnetic stratum, distributed over the surface of the substrate but being substantially transparent to the exposing radiation, which, for office copying, is visible light.

In use, the recording member is magnetized and placed over the surface of the document to be copied and then exposed to a high intensity controlled pulse of radiation from, for example, a xenon arc. The pulse of radiation thermally biases the magnetic elements of the recording member to a temperature below, but in the vicinity of the Curie temperature, and then image-wise raises the magnetic stratum into the temperature region of the Curie transition by light transmitted through the recording member then reflected selectively by the more reflective (white) areas of the document.

Recording members for use in reflex thermomagnetic copying of documents should preferably employ a hard magnetic stratum of relatively low Curie temperature. Chromium dioxide including modified chromium dioxides provide a range of Curie temperatures between about 75 and 150 C. and hence are particularly suited to this application.

The recording member must contain the magnetic material in agglomerates of suitable size in order that, when imaged as described above, the magnetized agglomerates will possess a suflicient magnetic field to attract magnetic toner or ink particles. On the other hand the recording member must be sufliciently transparent to transmit radiation for the reflex process i.e., a transmission of at least 2% and preferably 30 to 60% transmission.

The agglomerates of the present invention can be formed readily into recording members by making a suspension of the agglomerates in an adhesive binder and doctoring a single layer of the agglomerates over a suitable film. Preferably the binder is the same alkyd formulation employed to encapsulate and bind the magnetic particles. For this purpose the agglomerates need not be isolated from the suspending medium, but further quantities of alkyd resin can be added with or without removal of part of the suspending medium to form a moderately viscous pigment suspension. Films 10-15 microns thick containing l.42.2 grams/Sq. m. of chromium dioxide and transmitting 20% to 40% of visible light are readily prepared by this technique.

This invention is further illustrated by the following examples which are not, however, intended to ful y delineate the scope of this discovery.

EXAMPLES Example 1 Chromium dioxide, 60 =g., having an average particle size of 0.3g in length and 0.05/.L in diameter and a surface area of 16-18 sq.m./g., a coercivity of 405 oe., saturation magnetization 0 :81 e.m.u./g. and remanent magnetization, 41,:37 e.m.u./g. measured at ambient temperature, was placed in a 16 oz. wide-mouth jar with 132 ml. of a commercial aromatic solvent (Pansol RX5), 10 drops of oleic acid (U.S.P.) and 400 gm. of sand. The jar was shaken for 1 /2 hours on a commercial paint shaker, then filtered under pressure through a 50 mesh screen to remove the sand. g. of dispersion were obtained.

The dispersion (90 g.) thus obtained was mixed with 200 ml. of the aromatic solvent and 30 drops of sorbitan monolaureate added in a 32 oz. jar. This mixture was then shaken for 10 minutes under paint conditions.

15 g. of a commercial alkyd resin of the chinawood oil/linseed oil/ glycerol type having a medium oil length, a high molecular weight, an acid number of 34-44 and supplied as a 60% solids concentration (Du Pont RC- 288), was dissolved in 200 ml. of the aromatic solvent and poured into the dispersion and shaken for 1 hour on a paint shaker. The lid of the jar was then removed and the contents were stirred vigorously with a propellor type stirrer for 23 hours at ambient temperature.

Near the beginning and near the end of the reaction the slurry was examined microscopically and in each case found to contain agglomerates of chromium dioxide particles of 37/L diameter suspended in a clear homogeneous liquid.

The product of the reaction was placed over the poles of a strong permanent magnet and the pigment thus drawn to the bottom of the vessel. The clear supernatant liquid was decanted, then fresh solvent was added and shaken for 10 minutes under paint conditions. In this manner the product was washed three times with the aromatic solvent and twice with hexane. The product was then filtered on a Buchner funnel and washed with small quantities of hexane. Vacuum was pulled for 1 hour drawing dry nitrogen over the product, then the product was dried in a vacuum desiccator overnight. The product was finally screened through a 60 mesh screen.

The product consisted almost exclusively of 3-7;/. diameter agglomerates coated with the alkyd resin which were strong enough to resist shearing between glass plates when dispersed in a high viscosity oil.

The alkyd was removed from a small weighed sample by calcining the product wetted with concentrated H 50 over a Meker burner. The alkyd was burned off and the C10 was converted to Cr O From the weight of the residue the product contained 80.3% CrO and 19.7% by weight of the alkyl resin.

The above experiment was repeated except that the reaction time was 16 hours. The product was found to contain particles of 3-8n diameter and contained 14.6% by weight of the alkyd resin.

Examples 2-6 Using the general procedure described in Example 1, samples of the same chromium dioxide were coated and agglomerates cemented with various alkyd resins. The details of the experiments are shown in the accompanying Table I in which weights and volume are given for each 10 gm. of chromium dioxide employed. The solvent in each case was a blended highly aromatic solvent B.P.

175 C. to 225 C., aniline point 4 C.

TABLE I.ENCAPSULATION F CHROMIUM DIOXIDE AG GLOMERATES Product Alkyd Solvent, Time, \Vt. percent EX. Type Weight, g. vol., ml. hrs. Size, L alkyd A 2. 6 130 1 5 A 4. 8 130 05 3-10 25 B 4. 5 (i5 100 1 3 C 3.8 60 77 1-3 10 6 A 4. 5 60 16 3-8 16 Alkyd A is the same alkyd resin described in Example 1.

Alkyd B soya bean oil/ glycerol type, medium oil length, medium molecular weight, 60% solids acid No. 3-5 (Du Pont RC-453) Alkyd C linseed oil/glycerol type, medium oil length, low molecular weight, 83% solids, acid No. 3-5 (Du PontRC-141).

Example 7 A dispersion of fine particles of chromium dioxide containing 50 gm. CrO 100 gm. of an aliphatic oil [No. 470] and 10 gm. of oleic aicd was prepared by sand shaking as in Example 1. 56 gm. of alkyd resin (alkyd B of Example 4) was dissolved in 18 gm. of the above dispersion, then 0.05 N aqueous NaOH solution was added with vigorous stirring. After about 180 ml. of NaOH solution had been added and 2 hours had elapsed, the product was examined under a microscope and found to consist of small round particles of chromium dioxide encapsulated with alkyd resin.

Example 8 19 g. of carbonyl iron powder having an average particle size of 3a, a coercivity of 35 oe., a saturation magnetization of 199 e.m.u./ g. and a residual magnetization of 0.5 e.m.u./g. was placed in an 8 oz. glass jar together with 50 ml. of a commercial aromatic solvent and 5 drops of oleic acid. The jam was then shaken on a paint shaker for 1 /2 hours. 27 g. of a dispersion of chromium dioxide prepared according to the method of Example 1 and using the same materials as in that example was then added and the mixture stirred thoroughly with a propellor stirrer, then shaken on the paint shaker for minutes. A solution of 5 g. of alkyd resin A and 50 ml. of aromatic solvent was then added, and shaken on the paint shaker for 15 minutes at the end of which 10 drops of sorbitan monolaureate was added and shaking continued for 45 minutes. During this shaking period, a further 35 ml. of solvent was added as wash liquid. The resultant mixture was then stirred for 27 hours at ambient temperature with a propellor stirrer. The product was then worked up as described in Example 1.

The product was found to consist of agglomerates containing 91.1% by weight of magnetic material and 8.9% of alkyd resin, and having a size of 3-10 microns. Polished sections through the particles showed that they contained one or more of the relatively large iron particles surrounded by a chromium dioxide/ alkyd resin matrix. The magnetic properties of the particles obtained by the standard ballistic method showed a coercivity of 217 oe., saturation magnetization of 117 e.m.u./ g. and a remanent magnetization of 8 e.m.u./ g.

Example A.Preparation of a thermomagnetic recording member 22 g. of reaction slurry prepared by the method of Example 6 (taken prior to work-up) were allowed to settle and 10 g. of clear supernatant liquid was siphoned ofl. A further 8 g. of alkyd resin (Du Pont RC-288) and 10 drops cobalt octanoate was added to 10 g. of the described slurry and shaken 1 hour in a paint shaker.

Using a doctor knife with 1 mil clearance a film of the resultant magnetic paint was drawn over the surface of a 3 mil molecularly oriented polyethylene terephthalate film (Mylar), and then allowed to dry in air for several hours. The final dried coating was 0.4 mil thick. The optical transmission was 40%. The film contained about 1.7 g. of alkyd/Cr0 per square meter.

Portions of the above sheet were magnetized in DC. fields of 800-3500 0e. The films were then placed over a black and white photograph and exposed to pulses of light from a xenon flash tube in an integrating sphere subjected to energy stored in a condenser at pfClS. and 1200-1350 volts. The exposed films were then dipped in an ink containing a dispersed magnetic pigment. The magnetic pigment decorated the filrn to reproduce an image of the photograph.

Example B.Preparation of a magnetic ink 30 ml. of water, 0.6 g. of Lakeseal cleaner and 1.5 g. of alkyd/CrO agglomera'tes, prepared according to the method of Example 6 which had been magnetized in the source of isolation by separation using a magnetic field Were placed in a stainless steel beaker and dispersed by holding the beaker in an ultrasonically energized water bath for 10 minutes. The pH of the dispersion was about 9-9.5.

Magnetically imaged films were prepared by exposing magnetic recording tape which had been magnetized with an AC. recorded signal and exposed to a pulse of radiation from a xenon arc flash tube through a photographic negative. Imaged recording members were also prepared by reflex exposure using magnetic recording members containing chromium dioxide in a binder to fill an embossed line pattern on a substrate and exposing the premagnetized member in contact with a photographic positive as described in Example A. The imaged recording members were readily decorated with the above magnetic ink by contacting the member with the ink for /2 to 2 seconds, Washing with water and drying. The ink could 'be stripped and the image thus transferred from the recording members by contacting the decorated member with adhesive tape or by pressing the recording member on paper. Extremely black, crisp images were obtained having clean backgrounds.

Since obvious modifications and equivalents in the invention will be evident to those skilled in the chemical arts, I propose to be bound solely by the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

'1. Agglomerates of magnetic particles said particles having a maximum dimension of 0.01 to microns, said agglomerates having a size of from 1.5 to 30 times the maximum dimension of said particles, and containing at least 5% by Weight of a chromium oxide in which the average valence of the chromium is greater than 3, said particles being coated and bound together to form said agglomerates with an air-drying film-forming organic vehicle in an amount of from 5% to 25% by weight of said particles.

2. Agglomerates of claim 1 in which said air-drying film-forming organic vehicle is a drying alkyd resin.

3. Agglomerates of claim 2 in which said particles have a maximum dimension of from 0.1 to 2 4. Agglomerates of claim 3 in which said agglomerates have a substantially uniform size between about 2 and 3011..

5. Agglomerates of claim 4 in which said alkyd resin is present in an amount of from about to about by weight of said magnetic particles.

6. Agglomerates of claim 2 in which said magnetic particles consist essentially of chromium dioxide.

7. Agglomerates of claim 6 in which said particles have a maximum dimension of from 0.1 to 21.0.

8. Agglomerates of claim 7 in which said agglomerates have a substantially uniform size between 2 and 30 9. Agglomerates of claim 8 in which said alkyd resin is present in an amount of from about 10% to about 20% by weight of said magnetic particles.

10. A method for making agglomerates of particles of magnetic material said particles having a maximum dimension of from 0.01 to 5,, said agglomerates having a size of from 1.5 to 30 times the maximum dimension of said particles and containing at least 5% by weight of a chromium oxide, in which the average valence of the chromium is greater than 3, said particles being coated and cemented to form said agglomerates with an air-drying film-forming vehicle, which comprises (1) suspending said magnetic material in particulate form in an inert organic liquid medium and subjecting the resultant suspen sion to shearing agitation in the presence of an anionic surfactant and a non-ionic surfactant whereby agglomerates of magnetic particles are formed having substantially the desired size; (2) adding an air-drying filmforming organic vehicle to the resultant suspension of ag glomerates and agitating for a time sufficient to coat said particles and bind said agglomerates with a coating of said vehicle in an amount of from 10% to 20% by weight.

11. Method of claim 10 wherein said organic liquid medium is an aromatic hydrocarbon solvent.

12. Method of claim 11 wherein said air-drying filmforming organic meduim is a drying alkyd resin.

13. Method of claim 12 wherein said magnetic material consists essentially of chromium dioxide.

References Cited UNITED STATES PATENTS 2,384,579 9/1945 Vesce 260-22 2,879,246 3/1959 Jackson 260 3,082,171 3/1963 Shoemaker et al 252625 TOBIAS E. LEVOW, Primary Examiner A. P. DEMERS, Assistant Examiner U.S. Cl. X.'R. 96l.5; 252-62.1 

